A Machine Type Communication (MTC) User Equipment (UE or terminal) is also referred to as a Machine to Machine (M2M) user communication device, which is a main application form of a current internet of things.
Recently, due to high spectral efficiency of a Long-Term Evolution (LTE)/Long-Term Evolution Advance (LTE-Advance or LTE-A) system, more and more mobile operators select the LTE/LTE-A as an evolution direction of a broadband wireless communication system. LTE/LTE-A based MTC multi-type data services will be more attractive.
In the LTE system, a random access is a basic function, and a UE can be scheduled by the system to perform uplink transmission only after uplink synchronization with the system via a random access process. The random access in the LTE is divided into two forms namely a contention-based random access and a contention-free random access.
An initial random access process is a contention-based access process, which can be divided into four steps.
(1) A UE sends a preamble, and the UE randomly selects an available preamble to be sent.
(2) An evolved Node B (eNB, also referred to as an evolved base station) sends a Random Access Response (RAR). When the eNB detects a preamble sequence sent by the UE, a response will be sent over a Downlink-Synchronization Channel (DL-SCH), the response including: an index number of the detected preamble, time adjustment information for uplink synchronization, initial uplink resource allocation (used for sending a subsequent message 3), and a Temporary Cell Radio Network Temporary Identity (TC-RNTI). It will be decided whether the TC-RNTI is converted into a permanent C-RNTI in Step (4) (contention resolution). The UE needs to monitor an RAR message over a Physical Downlink Control Channel (PDCCH) by using a Random Access RNTI (RA-RNTI).RA-RNTI=1+t_id+10*f_id, 
where t_id refers to an index number of a first subframe of a Physical Random Access Channel (PRACH) for sending a preamble (0<=t_id<10),
f_id is a PRACH index in this subframe, i.e., a frequency domain position index (0=<f_id<=6), but there is only one frequency domain position for a Frequency Division Duplexing (FDD) system, and therefore f_id is always zero.
(3) The UE sends the message 3. After receiving the RAR message, the UE obtains uplink time synchronization and uplink resources. However, at this time, it cannot be determined that the RAR message is sent to the UE itself instead of other UEs. The preamble sequence of the UE is randomly selected from common resources, thereby making it possible for different UEs to send the same access preamble sequence over the same time-frequency resource. Thus, they will receive the same RAR via the same RA-RNTI. Moreover, the UE is unable to know whether other UEs make a random access by using the same resource. For this purpose, the UE needs to resolve such a random access contention via the subsequent message 3 and message 4.
(4) The eNB sends the message 4, namely a contention resolution message. If the UE receives the message 4 returned by the eNB and a UE Identifier (ID) carried therein conforms to an ID reported to the eNB in the message 3 within the time of a mac-Contention Resolution Timer, the UE considers that it wins this random access contention and the random access is successful, and sets the TC-RNTI obtained in the RAR message as an own C-RNTI. Otherwise, the UE considers that the random access is unsuccessful, and executes a random access retransmission process in accordance with the above-mentioned rule.
As for the contention-free random access, the preamble sent by the UE is notified by the eNB, uplink synchronization is completed via the first two steps, and a contention resolution process is not executed.
Future communication requirements for a huge number of machine devices are as follows. A random access concurrent transmission blocking rate is smaller than 0.1%, and the access density within 1 s to 10 s is not smaller than 10 UEs per square meter. So, at least tens of thousands of UEs are accessed to a micro cell within 1 s to 10 s. In order to meet this demand, even if UEs are uniformly accessed and each subframe can initiate a random access, at least hundreds of times of PRACH resources are needed in accordance with a random access mode in the related art. However, actually, the UEs are not uniformly accessed. Therefore, more resources may be needed. In a conventional LTE system, if one time-frequency resource receives 64 cyclic shifts of one preamble root sequence, resources are insufficient for a system having a bandwidth of 20 Mbps even though all bandwidths are used to send the PRACH.